1. Field of the Invention
The invention relates to optical fibers for use in communication networks.
2. Background of the Invention
Optical fibers are used to guide light with extremely small attenuation during propagation. These fibers are typically fabricated using different glasses characterized by low absorption and scattering losses for the wavelength range of the guided light. These fibers consist of a core region to which the light is confined and a cladding region that enables the light confinement. Most applications require fibers that support only the propagation of one fundamental transverse optical mode within the core region. Such fibers are called single mode fibers. Conventional single mode fibers include a step in the refractive index between core and cladding material as illustrated in FIG. 1 that shows the spatial profile of the refractive index. The best-known example of a single mode step index fiber is SMF-28, which was originally developed by Corning and is widely used. In this type of fiber light guiding is based on total internal reflection between core and cladding. To achieve this, the refractive index of the core material ncore has to be larger than the refractive index of the cladding material ncladding.
In order to achieve single transverse mode guiding in this type of fiber a characteristic fiber parameter; called V number, must satisfy the following criterion:V=(2π*rcore/λ)*[ncore2−ncladding2)0.5]<2.405  (1)where rcore is the radius of the core and λ the wavelength of light propagating in the fiber. The expression [(ncore2−ncladding2)0.5] is called the numerical aperture (NA) of the fiber.
The criterion (1) determines the design of single-mode step index fibers and limits the core size for optical mode propagation. For example, a fiber with rcore=4 μm has to have an NA smaller than 0.12 in order to become single mode for the optical communications wavelength around 1550 nm. Therefore, if the fiber is made of glass, the refractive index difference between core and cladding glass has to be smaller than 5*10−3. The technological limit for precise refractive index control of about 10−3 results in a lower limit for the NA of about 0.06 and, therefore, a maximum core radius of about 20 μm for step index fibers single mode that are single mode at near infrared wavelength.
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